I have a 2001 Frontier with 45k miles. My check engine light came on and I took it to the dealership. I had a fuel vacuum issue and a faulty O2 sensor. The truck was running fine. Dealership said that part of the motor had to be disassembled to replace the sensor. If it was not replaced it could cause the piston to prefire and possibly damage the engine. They replaced the sensor and said that no damage was done to the motor. It has cost me $875.00! Has anyone else had this issue. I thought an O2 sensor was a pretty simple replacement, also seems like low miles for the sensor to go bad.